The proposed research is directed toward improvement in the clinical durability of particulate reinforced polymer dental restorative materials. It is hypothesized that clinical durability is strongly influenced by the static and dynamic mechanical properties of the materials and that improvement in durability requires understanding of the response of mechanical behavior to conditions of the oral environment. It is proposed to study both commercial materials and a model system in which compositional variables will be limited to the nature of the reinforcing particles. Tensile properties, fracture toughness parameters, and fatigue behavior will be studied. Fatigue will be studied as functions of applied stress, stress intensity factor and surface condition. The effects of temperature and exposure to water will be quantified. Properties will be correlated with microstructures of the materials and attempts will be made to associate properties with available clinical wear data.